This application claims benefit of priority to Japanese application number JP 2001-77039 filed Mar. 16, 2001, the entire content of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to a motor control device which is reduced in size, is of lower cost, and wherein maintenance is facilitated.
2. Description of the Related Art
Conventionally, the following two systems are employed for the control devices of, for example, permanent magnet synchronous motors. Specifically:
(1) system wherein the position of the rotating magnetic field is detected and control of the driving inverter is performed in accordance therewith; and
(2) system wherein the output frequency of the inverter is controlled by an open loop, irrespective of the position of the magnetic field.
The former is sometimes called a brushless DC motor, since similar operation and characteristics are obtained to those of a DC motor.
The latter has similar control characteristics to the voltage/frequency control (V/f) of an induction motor, but in the case where a forced cooling system thereof is employed loss of synchronization can occur.
Consequently, owing to the difficulty of obtaining large torque at low rotational speed, this system is only employed in special applications, such as winders of fiber machinery.
FIG. 1 is a block diagram illustrating the basic control layout of a permanent magnet synchronous motor of this type.
As shown in FIG. 1, a permanent magnet synchronous motor comprises a permanent magnet synchronous motor body 1, a voltage source PWM inverter (hereinbelow simply called an inverter) 2 for converting DC power to AC power and supplying it to permanent magnet synchronous motor 1, pole position detector 3 for determining the phase of the applied voltage or current of permanent magnet synchronous motor 1 and current controller 4.
In order to perform speed control, there are further added thereto a speed controller 5 for this purpose and a rotary speed detector 3.
Further, when position control is to be performed, a position controller 6 and position detector 3 are added.
Recently, in addition, as a control device for a permanent magnet synchronous motor of this type, a system has been proposed in which control is performed using vector control.
FIG. 2 is a functional block diagram illustrating an example layout of a control device for a prior art permanent magnet synchronous motor using such vector control, elements which are the same as elements in FIG. 1 being given the same reference symbols.
In FIG. 2, current control means (unit) 7 inputs d axis current instruction and q axis current instruction IdRef, IqRef and the actual values Id, Iq of the d axis current and q axis current that are output from current co-ordinate conversion means (unit) 9, finds by calculation the d axis voltage instruction and q axis voltage instruction VdRef and VqRef and outputs these.
Voltage instruction co-ordinate conversion means (unit) 8 inputs the d axis voltage instruction and q axis voltage instruction VdRef and VqRef that are output from current control means (unit) 7 and the position detection value 6 of the motor rotor that is output from rotor position sensor 10, and finds by calculation and outputs the three-phase voltage instructions Vu, Vv and Vw of inverter 2.
It should be noted that, although the symbols such as for example Vu, Vv and Vw and other symbols indicated in FIG. 2 should properly be represented as vector quantities, for convenience, they are represented as scalar quantities.
Current co-ordinate conversion means (unit) 9 inputs the two phase current detection values Iu, Iw of the three phases and rotor position detection value xcex8 that is output from rotor position sensor 10, and finds by calculation and outputs the actual values Id, Iq of the d axis current and q axis current, which are their values on the d and q co-ordinate axes.
However, in cases where the motor output torque of for example a permanent magnet motor or reluctance motor is to be controlled with high precision and high speed by a control device using vector control as described above, a rotor position sensor 10 must be mounted in order to enable inflow of current in accordance with the position of the motor rotor.
However, rotor position sensor 10 was of comparatively large volume, which made the device bulky and tended to restrict possibilities regarding its installation; furthermore, possible causes of faults such as the difficulty of leading out a control lead for transmitting output from the rotor position sensor 10 to the main unit of the control device and also disconnection etc were increased, raising costs and making maintenance more difficult.
In a permanent magnet motor, it is possible to ascertain the position of the motor rotor indirectly by detecting the motor back e.m.f. (electromotive force (BEMF) or counter electromotive force (CEMF)) caused by the magnetic flux of the permanent magnet during rotation, but in operating conditions of zero rotational speed, in which in principle no back e.m.f. is generated, there is the problem that it is not possible to thereby ascertain the position of the motor rotor.
An object of the present invention is to provide a motor control device wherein the position of the motor rotor can be ascertained with high precision by a straightforward device construction without employing a rotor position sensor and reduction in size, reduction in costs and facilitation of maintenance can be achieved.
In order to achieve the above object, the present invention is constructed as follows. Specifically, a motor control device comprising:
a motor rotor having magnetically protruding polarity;
an inverter that converts DC power to AC power; and
a control unit that supplies the output from this inverter to the motor rotor, further comprises:
a PWM pattern generating means (unit) that calculates and outputs a PWM pattern such as to output a pulse synchronized with the period of pulse width modulation (PWM) of the inverter and depending on the inferred position value of the motor rotor;
a current co-ordinate conversion means (unit) that, using the inferred position value of the motor rotor converts the three-phase current of the motor to actual values of the d axis current and q axis current, which are values on the d and q co-ordinate axes, wherein q axis is direction of the protruding polarity of the motor rotor, and d axis is direction at right angles to the direction of protruding polarity of the motor rotor;
a d and q axis rate of current change calculating means (unit) that respectively calculate and output the rates of change with time of the d axis current and q axis current actual values, which are the outputs from the current co-ordinate conversion means (unit), synchronized with the PWM pattern which is the output from the PWM pattern generating means (unit); and
a motor rotor position inferring means (unit) that infers and calculates the position of the motor rotor, using the PWM pattern which is output from the PWM pattern generating means (unit) and the d axis rate of current change and q axis rate of current change which are output from the d and q axis rate of current change calculating means (unit) and outputs this as the inferred position value of the motor rotor.
Consequently, in a motor control device according to the present invention, thanks to the provision of means as described above, a pulse synchronized with the period of pulse width modulation (PWM) of the inverter and depending on the inferred value of the position of the motor rotor is applied as a PWM pattern to the motor and as a result the difference of magnetic resistance of the motor rotor, which can be inferred by observing the rate of change with time of the d axis current and q axis current on the d and q axis co-ordinates of the current flowing through the motor winding is detected and the position of the motor rotor can be thereby inferred without using a rotor position sensor.
In particular, since a pulse synchronized with the PWM pattern is applied and the rate of change of current of the d axis current and q axis are detected synchronously with the PWM pattern, error in inferring the motor rotor position caused by current harmonics in PWM switching can be reduced.
In this way, the position of the motor rotor can be ascertained without employing a rotor position sensor; reduction in size, reduction in costs and increased ease of maintenance can thereby be achieved.